linux-kernel - Re: whose job is it to include various header files?

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Date:	Fri, 14 Mar 2008 00:53:17 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: whose job is it to include various header files?

On 13/03/2008, Robert P. J. Day <rpjday@...shcourse.ca> wrote:
>
>   more a philosophy question than anything but, while poking around
>  the percpu stuff today, i noticed in the header file linux/percpu.h
>  the opening snippet:
>
>   #include <linux/preempt.h>
>   #include <linux/slab.h> /* For kmalloc() */
>   #include <linux/smp.h>
>   #include <linux/string.h> /* For memset() */
>   #include <linux/cpumask.h>
>   ...
>
>  hmmm, i thought to myself (because that's how i refer to myself), i
>  wonder why this header file is including headers for kmalloc() and
>  memset() when this header file makes no reference to those routines.
>  let's see what happens if i remove them and:
>
>   $ make distclean
>   $ make defconfig    [x86]
>   $ make
>
>  ... chug chug chug ...
>
>   CC      arch/x86/kernel/nmi_32.o
>  arch/x86/kernel/nmi_32.c: In function 'check_nmi_watchdog':
>  arch/x86/kernel/nmi_32.c:81: error: implicit declaration of function 'kmalloc'
>  arch/x86/kernel/nmi_32.c:81: error: 'GFP_KERNEL' undeclared (first use in this function)
>  arch/x86/kernel/nmi_32.c:81: error: (Each undeclared identifier is reported only once
>  arch/x86/kernel/nmi_32.c:81: error: for each function it appears in.)
>  arch/x86/kernel/nmi_32.c:81: warning: assignment makes pointer from integer without a cast
>  arch/x86/kernel/nmi_32.c:118: error: implicit declaration of function 'kfree'
>  make[1]: *** [arch/x86/kernel/nmi_32.o] Error 1
>  make: *** [arch/x86/kernel] Error 2
>  $
>
>   ok, now i know.  but that means, of course, that nmi_32.c is
>  invoking kmalloc() without ever having included the necessary header
>  file for it -- it's just inheriting that from linux/percpu.h.
>
>   doesn't that (sort of) violate the kernel coding style?  if a file
>  somewhere needs the contents of some header file, isn't it that file's
>  responsibility to explicitly include it, and not quietly realize it's
>  getting it from elsewhere?
>
I agree with you completely. A file should explicitly include headers
for the stuff it uses and not rely on implicit includes done
elsewhere. Cleaning that up is going to touch a lot of files though
for no real short term gain (there is a long term gain of
maintainability though), so it's going to be a loveless job :(

-- 
Jesper Juhl <jesper.juhl@...il.com>
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